Embodiments of the present invention relate to a programmable resistor array for a continuous time, programmable gain amplifier (PGA) filter.
Reduced power supply voltages and gate dielectric thickness limit overdrive voltage available for metal oxide semiconductor (MOS) switches in rail-to-rail circuit operation. A maximum gate voltage limit is imposed by gate oxide integrity (GOI) reliability, and a minimum gate voltage limit is imposed by a requirement for linear operation of the MOS switch. CMOS transmission gate switches are generally unsuitable for switching operations due to the nonlinear region near the center of the power supply voltage range between p-channel and n-channel threshold voltages. Referring to FIGS. 1A and 1B, Kim et al., “Body Effect Compensated Switch for Low Voltage Switched-Capacitor Circuits,” IEEE International Symposium on Circuits and Systems, Vol. 4, IV-437, IV-440 (2002), disclose a compensated switch for low voltage switched-capacitor circuits. The improved circuit is shown at FIG. 1A with compensated switch SW circled. The concept of operation is illustrated at FIG. 1B and described at pages IV-438 through IV-439. In operation, the circuit first discharges the gate of SW by switch S5 and charges capacitor CB to VDD by switches S1 and S3. Then switches S1, S3, and S5 are turned off, and switches S2 and S4 are turned on to supply a compensated gate voltage to switch SW. Although this circuit performs well for switched-capacitor applications such as a duty-cycled environment where the switch does not need to remain on continuously, it is incompatible with continuous time operation.